P-values in Minitab can be found from the command line.  If I want to
find a single p-value for a 2^3 experiment with 3 reps (that's 16
degrees of freedom), I would type in the Session window:

cdf 1.19;
t 16.

This actually gives me the area to the left of 1.19.  You should
subtract this answer from 1 and then double it to get the p-value.

If you want a critical value for the same situation, you would type in
the Edit>Command Line Editor window:

invcdf .975;
t 16.

Any effect's T statistic whose absolute value was larger than the answer
provided by Minitab would be significant.  If you want to find p-values 
for several effects simultaneously, enter their T statistics into a 
column (e.g., C1) and type:

let c1=abs(c1)
cdf c1 c2;
t 16.
let c3=2*(1-c2)

That last step computes p-values for you.